This invention relates to the recovery of liquid and gaseous products from oil shale. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a formation comprising marlstone deposit interspersed with layers of an organic polymer called "kerogen" which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the deposit containing kerogen that is called "oil shale" herein, and the liquid product is called "shale oil".
One technique for recovering shale oil is to set up a retort in a subterranean oil shale deposit. The shale within the retort is fragmented and the shale at the top of the retort is ignited to establish a combustion zone. An oxygen containing gas is supplied to the top of the retort to sustain the combustion zone, which proceeds slowly down through the fragmented shale in the retort. As burning proceeds, the heat of combustion is transferred to the shale below the combustion zone to release shale oil and gases therefrom in a retorting zone. Thus, a retorting zone moves from top to bottom of the retort in advance of the combustion zone, and the resulting shale oil and gases pass to the bottom of the retort for collection.
In preparation for the described retorting process, it is important that the shale be fragmented, rather than simply fractured, in order to create high permeability; otherwise, too much pressure is required to pass the gas through the retort. Known methods of creating such high shale permeability call for mining large volumes of the oil shale prior to fragmentation. This is objectionable in two respects. First, mining the shale and transporting it to the ground level are expensive operations. Second, the mined shale is excluded from the in situ retorting process, thus reducing the overall recovery of shale oil from the retort.